Friday, November 30, 2007

The vaults resolved

What are vaults? These 50 nm-long hollow capsules are made of almost 100 proteins, plus RNA components, and rank among the largest supramolecular structures found in eukaryotic cells. Yet their function is still unknown. They could have a function in the immune system, in tumour drug resistance, or in nucleus-cytoplasm trafficking. There is also a practical reason for studying vaults. This kind of molecular spaceship could one day be engineered to carry pharmacological agents in the cells.

Characterize molecular complexes as gigantic as vaults is one of the most daunting tasks for science today. They lie in a "dark" length scale. Smaller molecules are easy often to crystallize and thus to get their structure solved atomically thanks to X ray diffraction. Larger structures can be characterized with microscopy. Vaults lie in the middle: too large to efficiently crystallize, too small to be solved with conventional microscopy. The three-dimensional general barrel shape of vaults has been structurally described by mean of cryo-TEM at low resolution. A hollow symmetric shell (able to encapsulate entire ribosomes), with two protrudring caps, was revealed, but that pretty was it.

Now, Daniel H. Anderson and others have resolved the structure of the vault shell protein at 0.9 nm resolution. The picture is still fuzzy compared to the atomic resolution of many protein crystal structures, but it is an enormous step forward compared to the previous structures. The vault can now be understood in terms of the assemby of the individual molecules that compose it. It is a beautiful, daunting mosaic of intertwined protein chains.

You can read the full paper on PLoS Biology.

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